In the manufacture and use of switch boxes of the type which have receptacles to receive plugs and which are designed to handle relatively high currents, such as 30 amperes to 60 amperes or above, it is considered unsafe to allow the switch to be moved to its ON position in the absence of a properly inserted plug. Attempting to insert or remove a plug into or from an energized receptacle, especially with a load connected to the plug, can result in arcing between the plug and receptacle with damage to the components as well as a safety hazard to personnel. To prevent this occurrence, switches housed with receptacles (as distinguished from those which are permanently wired into a system) are commonly provided with some type of interlock.
One type of switch housing with an interlock is shown in German patent document 23 42 805. As shown therein, a switch handle is provided with a non-circular coupling to directly operate a conventional switch. A gear train engages teeth on the switch handle, one of the gears having a locking relationship with a spring-urged slider so that the gear train is not rotatable unless the slider is moved to an unlooking position. The slider is adjacent a receptacle shaped and dimensioned to receive a plug of the type having a protruding cam. When the plug is properly inserted, the cam moves the slider and releases the gears so that they can turn, allowing the switch to be turned on after the plug is in. Also, a latching member rotates with the gears to a position behind the cam, preventing extraction of the plug until the switch handle has been rotated to the OFF position.
While this mechanism is functional, it is possible to defeat the mechanism and it is also subject to malfunction. Because the handle is connected to a shaft which directly operates the switch, it is possible to force the handle and the switch, causing the gears to deform or break and jump out of their normal meshing relationship. That kind of interlock mechanism thus depends on the integrity of the gear train for its effectiveness.
It is also possible to defeat that mechanism by inserting a common probe type of tool, such as a screwdriver, an ice pick or a pencil, to push the latch and begin the rotation of the gear train, extract the probe and then continue the rotation until the switch is operated to the ON position, thus defeating the interlock mechanism.